The present invention relates to frequency discriminators and more particularly to means for improving the noise rejection capability of frequency discriminators used in certain distortion analyzers.
Various methods have been devised for analyzing the distortion of an audio frequency system. One of these tests is the total harmonic distortion plus noise test. Total harmonic distortion plus noise is measured by first applying a pure sinewave to the input of the system to be tested. The system output is then applied to a notch filter to remove the fundamental frequency. The remaining signals representing the distortion and noise introduced by the system under test are measured and displayed as a percentage of the total signal.
Heretofore distortion analyzers designed to perform the above measurements had only partly automatic tuning at best. What is required is a frequency discriminator which measures the frequency of the input signal and tunes the notch filter to the proper band. Also, the frequency discriminator must be immune to high levels of noise such as those present in SINAD testing. The term SINAD is an abbreviation for "signal+noise+distortion to noise+distortion ratio" expressed in db. The test procedure for SINAD may be found in EIA STANDARD RS-204-A "Minimum Standards for Land Mobile Communication FM or PM Receivers, 25-470 MHz", Electronic Industries Association, copyright 1972.